As a conventional turning portion structure of an industrial robot, one described in the following patent document 1 is known, for example.
Patent Document 1: JP-A-9-57678
This turning portion structure of an industrial robot is comprised of a fixed member; a rotating member; a reduction gear of an eccentric differential type which is disposed between the fixed member and the rotating member, has a plurality of crankshafts, reduces the speed of the rotational driving force imparted to the crankshafts and transmits it to the rotating member so as to rotate the rotating member; a motor mounted on the rotating member; a first transmitting member fixed to an output shaft of the motor; a second transmitting member fixed to a particular one of the crankshafts and adapted to receive the rotational driving force directly from the first transmitting member so as to rotate the particular crankshaft; a cylindrical gear which rotates by receiving the rotational driving force from the second transmitting member; and an external gear fixed to the crankshafts other than the particular crankshaft and adapted to mesh with the cylindrical gear to thereby rotate the crankshafts.